1. Field of the Invention
The present invention relates in general to a channel equalizer for a digital television (TV) receiver, and more particularly to a channel equalizer for a digital TV receiver in which a filtering coefficient converging on an optimum value is obtained for a firstly selected channel when an input signal of the firstly selected channel is filtered, the obtained filtering coefficient is stored and the stored filtering coefficient is used as an initial coefficient for filtering the input signal when the firstly selected channel is again selected.
2. Description of the Background Art
Generally, a channel equalizer is provided in a receiver to process a transmission signal in the opposite manner to a channel characteristic to remove an inter-symbol interference of the transmission signal due to a multi-path thereof or a non-linearity of a transmission channel.
Such a channel equalizer must essentially be provided in a digital TV receiver which receives digital data transmitted thereto. Namely, in the digital TV receiver, the channel equalizer is required to remove a ghost due to a multi-path of a digital TV broadcasting signal from a broadcasting station which results from obstacles such as buildings, mountains and etc. The channel equalizer is also required to remove an NTSC interference in which an NTSC signal of the same frequency band is mixed with the digital TV broadcasting signal.
Referring to FIG. 1, there is shown a block diagram of a conventional channel equalizer applied to a digicipher system. The digicipher system performs a convolutional encoding operation for data in the unit of 4 bits to output 5-bit data by converting one least significant bit (LSB) into two bits. Then, the digicipher system transmits the 5-bit data through trellis coded modulation (TCM) and 32 quadrature amplitude modulation (QAM).
As shown in FIG. 1, the channel equalizer comprises a finite impulse response (referred to hereinafter as FIR) filter 1 for filtering an input signal V.sub.k, an error detector 3 for detecting a difference between the input signal V.sub.k and an output signal I.sub.k from the FIR filter 1, and a coefficient update calculator 2 for updating a filtering coefficient C.sub.k for the filtering of the input signal V.sub.k in the FIR filter 1 in response to the input signal V.sub.k and an output signal .epsilon..sub.k from the error detector 3 and outputting the updated filtering coefficient C.sub.k to the FIR filter 1.
The operation of the conventional channel equalizer with the above-mentioned construction as shown in FIG. 1 will hereinafter be described.
The FIR filter 1 performs the filtering operation for the input signal V.sub.k in the form of a complex number by multiplying it by the updated filtering coefficient C.sub.k from the coefficient update calculator 2. The output signal I.sub.k from the FIR filter 1 can be expressed as follows: ##EQU1## Here, V.sub.k-j, C.sub.j and I.sub.k are complex numbers because the digicipher system performs the signal transmission in the form of 32QAM.
The error detector 3 calculates an error or the difference between the input signal V.sub.k and the output signal I.sub.k from the FIR filter 1.
The coefficient update calculator 2 includes a coefficient storage unit 2a. The coefficient update calculator 2 updates the filtering coefficient C.sub.k for the filtering of the input signal V.sub.k in the FIR filter 1 every time on the basis of the following equation: EQU C.sub.j,k+1 =C.sub.j,k +.DELTA..epsilon..sub.k V.sub.k-j EQU (j=-k.sub.1, -k+1, . . . , 0, 1, . . . k.sub.2)
Here, .DELTA. is a predetermined step size, .epsilon..sub.k is the error detected by the error detector 3 and k is an iteration number. Whenever the input signal is received, the update operation is iteratively performed by the coefficient update calculator 2 so that a set of coefficients C.sub.-k1, C.sub.-k1-1, . . . , 0, 1, . . . C.sub.k2 can be updated. The updated coefficients are applied to the FIR filter 1 and also stored into the coefficient storage unit 2a for the next iterative operation. As seen from the above equation, the updated filtering coefficient C.sub.j,k+1 for the present input signal V.sub.k-j is obtained by adding the filtering coefficient C.sub.j,k for the previous input signal to a value obtained by multiplying the step size .DELTA., the error .epsilon..sub.k and the present input signal V.sub.k-j.
Referring to FIG. 2, there is shown a block diagram of a conventional channel equalizer applied to a digital spectrum compatible high definition television (DSC-HI)TV). As shown in this drawing, the channel equalizer comprises a feedforward FIR filter 4 for filtering an input signal V.sub.k, a slicer 7 for slicing an output signal I.sub.k of the channel equalizer to detect a value between predetermined levels therefrom, a feedback FIR filter 6 for filtering an output signal I.sub.k from the slicer 7, an error detector 8 for detecting a difference between the output signal I.sub.k of the channel equalizer and the output signal I.sub.k from the slicer 7 to detect an error between the input signal V.sub.k and the output signal I.sub.k of the channel equalizer, an adder 5 for adding an output signal from the feedforward FIR filter 4 to an output signal from the feedback FIR filter 6 to provide the output signal I.sub.k of the channel equalizer, and a coefficient update calculator 9 for updating a filtering coefficient C.sub.k in response to the input signal V.sub.k, the output signal I.sub.k from the slicer 7 and an output signal .epsilon..sub.k from the error detector 8 and outputting the updated filtering coefficient C.sub.k to the feedforward FIR filter 4 and the feedback FIR filter 6.
The operation of the conventional channel equalizer with the above-mentioned construction as shown in FIG. 2 will hereinafter be described.
The output signal I.sub.k from the adder 5 can be expressed as follows: ##EQU2## It can be seen from the above equation that the output signal I.sub.k from the adder 5 is obtained by adding the output signal ##EQU3## from the feedforward FIR filter 4 and the output signal ##EQU4## from the feedback FIR filter 6. I.sub.k-j is the output value from the slicer 7 which slices the output signal I.sub.k-j of the channel equalizer.
The feedforward FIR filter 4 performs the filtering operation for the input signal V.sub.k by multiplying it by the updated filtering coefficient C.sub.k from the coefficient update calculator 9. The feedback FIR filter 6 performs the filtering operation for the output value I.sub.k from the slicer 7, which slices the output signal I.sub.k of the channel equalizer, by multiplying it by the updated filtering coefficient C.sub.k from the coefficient update calculator 9.
The coefficient update calculator 9 includes a coefficient storage unit 9a. The coefficient update calculator 9 updates the filtering coefficient C.sub.k whenever the input signal is received. The filtering coefficient for the feedforward FIR filter 4 is updated on the basis of the following equation: EQU C.sub.j,k+1 =C.sub.j,k +.DELTA..epsilon..sub.k V.sub.k-j
where, EQU j=-k.sub.1, -k.sub.1+1, . . . , 1, 0
As seen from the above equation, the updated faltering coefficient C.sub.j,k+1 for the present input signal is obtained by adding the filtering coefficient C.sub.j,k for the previous input signal to a value obtained by multiplying the step size .DELTA., the error .epsilon..sub.k and the present input signal V.sub.k-j. Here, j=-k.sub.1, -k.sub.1+1, . . . , 1, 0 is for removing an interference appearing as a part of a transmission signal is earlier inputted to the receiver than that under a normal condition.
The filtering coefficient for the feedback FIR filter 6 is updated on the basis of the following equation: EQU C.sub.j,k+1 =C.sub.j,k +.DELTA..epsilon..sub.k I.sub.k-j
where, EQU j=1, 2, 3, . . . , k.sub.2
As seen from the above equation, the updated filtering coefficient C.sub.j,k+1 for the present input signal is obtained by adding the filtering coefficient C.sub.j,k for the previous input signal to a value obtained by multiplying the step size .DELTA., the error .epsilon..sub.k and the presently sliced signal I.sub.k-j . Here, j=1, 2, 3, . . . , k.sub.2 for removing an interference appearing as a part of a transmission signal is later inputted to the receiver than that under a normal condition because of reflection from obstacles, for example.
The error detector 8 calculates the error between the input signal V.sub.k and the output signal I.sub.k of the channel equalizer. The output signal .epsilon..sub.k from the error detector 8 can be expressed as follows: EQU .epsilon..sub.k =I.sub.k -I.sub.k
In the conventional channel equalizer applied to the digicipher system or the DSC-HDTV, where the step size .DELTA. is fixed and an adaptive least mean square (LMS) algorithm is employed to control continuously the filtering coefficient so that a mean square error between the input signal and the output signal of the adaptive filter can reach a minimum value, the calculation mount is not large, but a considerable time is required in obtaining a filtering coefficient converging on an optimum value for removal of a noise such as the ghost. Namely, a convergence speed is low. For this reason, a picture of a changed channel appears on a screen after the lapse of a considerable time from a channel change time point in a digital TV receiver in the case where many multi-path components are present.